This disclosure relates to gamma ray well logging tools and, more particularly, to composition-matched tools that produce substantially only one neutron-derived tool noise background spectrum.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
A variety of downhole tools may be used to determine the properties of a geological formation surrounding a well. Some downhole tools, known as “neutron-gamma spectroscopy” tools, emit neutrons into the geological formation and detect the spectra of gamma rays that result when the neutrons interact with the elements of the formation. Interactions between the elements of the formation and the neutrons may produce gamma rays in at least two ways: by inelastic scattering and by neutron capture. Inelastic scattering occurs when fast neutrons collide with elements of the formation, which may result in the emission of one or more gamma rays. Neutron capture occurs when lower-energy thermal or epithermal neutrons are captured by the nuclei of elements of the formation, which also may result in the emission of one or more gamma rays. In either case, the various energies of the resulting gamma rays may be detected by gamma ray detectors in the downhole tool to obtain gamma ray spectrum measurements. The spectra of gamma rays obtained at various depths in the well may be used to ascertain a variety of different well properties.
Although many gamma rays are generated through interactions between the emitted neutrons with the elements of the formation, some gamma rays may be generated through interactions of the emitted neutrons with the materials of the downhole tool itself. These gamma rays produce a noise background that may reduce the signal-to-noise ratio (SNR) of the downhole tool spectroscopy measurement. Indeed, neutron interactions with the material of the downhole tool occurring near or within the gamma ray detector itself may substantially increase the amount of unwanted background noise. Since these noise-producing neutron interactions occur close to or inside the detector, the detection probability, even in the presence of a low neutron flux, may be high.
These noise-producing neutron interactions may be partially accounted for during processing, but doing so affects the precision and/or accuracy of the gamma ray spectroscopy measurement. In addition, some of the elements of the formation may also be present in the downhole tool. As such, removing this background noise may further involve attempting to distinguish the unwanted contribution of the background noise from the downhole tool and then accounting for this noise. These additional computations and decisions may further reduce the precision and/or accuracy of the measurement.